A. Field of the Invention
This invention relates to a robot and method for automatically bending orthodontic archwires, retainers, or other orthodontic or medical devices into a particular shape.
B. Description of Related Art
In orthodontics, a patient suffering from a malocclusion is treated by affixing brackets to the surface of the teeth and installing an archwire in the slots of the brackets. The archwire and brackets are designed to generated a customized force system that applies forces to teeth, by which individual teeth are moved relative to surrounding anatomical structures into a desired occlusion. There are two approaches to designing an appropriate force system for a patient. One is based on a flat archwire and customized brackets, e.g., Andreiko et al., U.S. Pat. No. 5,447,432. The other is based on off-the shelf brackets and designing a customized archwire that has complex bends designed to move or rotate the teeth in the desired direction. Traditionally, the latter approach has required manual bending of the archwire by the orthodontist.
Machines for bending orthodontic archwires have been proposed in the prior art. Andreiko et al. describes an apparatus that takes a straight archwire and imparts a simple planar arcuate curvature to the wire. The wire is customized in the sense that the shape of the arc is designed for a particular patient, but the wire bending apparatus described in Andreiko et al. is limited to a customized bracket approach to orthodontics. In particular, the Andreiko et al. wire bending apparatus cannot produce any complex and twists bends in the wire, e.g., bends requiring a combination of translation and rotational motion.
The patent to Orthuber et al., U.S. Pat. No. 4,656,860 describes a bending robot for bending archwires. A robot as described in the '860 patent was developed by the predecessor of the assignee of the present invention and used experimentally for several years, but never widely commercialized. The robot consisted of two characteristic design features: a bending cone that could move forwards and backwards to bend the wire, and a rotating cone that could twist the wire. As such, it could only apply torque or bends over the two main axes of a cross section of a rectangular shaped wire. Since the portion of the wire extending beyond the cone is free and unconstrained, the robot had no control as to the effective deformation of the wire. Additionally, a series of three twists and two bends were typically required by a robot in accordance with the '860 patent to shape an archwire so that it would fit in the slots of two adjacent brackets. This series of twists and bends required as much as 5 mm of wire length between adjacent brackets. This length of wire is greater than that available for closely spaced teeth, such as the lower front teeth. To avoid this situation, the robot bent a twisted portion of the wire, which provoked uncontrolled rotational motion in the wire.
The design of the '860 patent also has other shortcomings: it provides no means for measuring forces imparted by the wire since one end of the wire is free and the wire is gripped immediately below the bending point. The robot had no effective feedback mechanism for detecting how the wire in fact was bent after a particular bending or twisting operation was performed. As the free end of the wire is not constrained or held in any manner, there is no ready way to heat the wire as it is being bent in order to fix the shape of the bend in a wire made from a shape memory material. Consequently, shape memory alloy wires made with the '860 patent were subject to a separate heating treatment in a separate thermal device.
The present invention presents a substantial improvement to the robot of the '860 patent. The invention also provides for much greater flexibility in the design and manufacture of archwires than that disclosed by the Andreiko et al. patent. In particular, the present invention enables the manufacture of custom, highly accurate orthodontic archwires. Such wires are ideally suited to an archwire-based orthodontic treatment regime based on standard, off-the-shelf brackets. The invention is also readily adaptable to bending other medical devices, including implants such bone fixation plates, prostheses, orthotic devices, and even surgical tools.